The present invention is directed towards improved relay switching apparatus and, more particularly, to relay switching apparatus which is used to provide a physical air gap between a power source and a load when power is removed from the load.
Relay switches of the foregoing type are especially important in connection with household appliances where it is often possible for the operator of the appliance to come into contact with the power lines coupling the power source to the load. One such application is the power switching of theCalrod.RTM. elements for the surface unit of an electric range. In a typical unit, the user can turn the surface unit off and clean under the surface unit with a wet cloth. During such cleaning operations, it is possible to come into contact with the power leads supplying power to the surface unit. This constitutes a safety hazard unless a physical air-gap-disconnect is provided between the power lines feeding the surface unit and the power supply, when the unit is off.
In order to produce such an air gap, the prior art has provided a relay in series with the power line supplying power to the load. Whenever the surface unit is shut off, the relay is opened to provide a physical air gap between the power source and the power line feeding the surface unit. In order to prolong the life of the relay, typical prior art devices shunt the relay with a triac which is turned on by a timing and control circuit whenever the relay is to be opened or closed. As a result, the current which would normally flow through the relay is shunted by the triac and the relay is not subjected to both high currents and high voltages while it is switching states and therefore need not be capable of withstanding such power levels during the transition period. This makes possible the use of less expensive relays and lengthens the life of the relays. Once the relay has been opened or closed, the triac is disabled and power is either applied to, or cut off from, the load, depending upon the state of the relay.
While the foregoing apparatus has been found to be generally useful, it has two major drawbacks: (1) it requires a controlled gate supply to the triac which is of line voltage; and (2) triacs have a certain amount of leakage current in the off state, thereby enabling a certain amount of current to flow through the power leads even when the contact is open and the triac is off. As a result of the latter drawback, there is a chance that the user will receive a shock when contacting the power leads. For this reason, Underwriters Laboratories consider relay switching systems using triacs to be unsafe.